Three component magnetic developer for electrophotographic purposes and method for using it



y 26, 1966 G. GOURGE 3,262,806

THREE COMPONENT MAGNETIC DEVELOPER FOR ELEGTROPHOTOGRAPHIG PURPOSES ANDMETHOD FOR USING IT Filed Dec. 13, 1962 IN V EN TOR. Gerfiam Gazzrye' ,4TTOR/V United States Patent 3,262,806 THREE COMPONENT MAGNETIC DEVELOPERFOR ELECTROPHOTOGRAPHIC PURPOSES AND METHOD FOR USING IT Gerhard Gourg,Wiesbaden, Germany, assignor to Azoplate Corporation, Murray Hill, NJ.

Filed Dec. 13, 1962, Ser. No. 244,251 Claims priority, applicatignGermany, Dec. 16, 1961,

34 Claims. (cl. 117-175 The toners commonly used in electrophotographycomprise natural or synthetic resins in finely divided powder form. Whenused for the development of latent electrostatic images, such a toner ismixed with somewhat larger particles-of a carrier which generallycomprises an inorganic material, e.g. iron powder.

Iron powder is particularly effective as a carrier as it It has beensuggested to use toners consisting of two components which haveapproximately the same grain sizes. One toner component acquires apositive charge during handling and the other toner component acquires anegative charge. The use of these toners gives particularly desirableresults with respect to freedom from background and sharpness of imagesproduced therewith.

It is, however, desirable for the advantages of a magnetic system ofapplication to be combined with the good properties of these last-namedtoners. In this, however, there have been considerable difficultiesbecause reciprocal charging or reversal charging of the three componentsdoes not always give reproducible results.

The object of the present invention is a method for developing latentelectrostatic images by means of a developer consisting of a carrier andtoner, said developer being characterized in that the carrier consistsof a magnetically excitable powder which is mixed with a tonerconsisting of at least two components of about equal grain sizes atleast one of which accepts a positive charge and the other a negativecharge and that this developer is taken up by a magnetic vdevice whichserves as an applicator system for the developer and, simultaneously, asa bias electrode.

The present invention is further concerned with a material forperforming the above described process, said material consisting of acarrier and a toner and being characterized in that the carrier consistsof a magnetically excitable powder and the toner consists of at leasttwo components of about equal grain sizes one of which accepts apositive charge and the other a negative charge.

The iron powder in the developer according to the invention does notparticipate with the toner components in the triboeiectri-c charging,but acts, in conjunction with a suitable magnetic applicator, as anapplicator system and, if desired, also as a bias electrode.

The developer according to the invention enables a latent electrostaticimage to be developed in such a way that fine lines and large solidareas are simultaneously given perfect development. At the same time, animage practically completely free of background is obtained.

The particle size of the iron powder is normally between 50 and 400 andpreferably between 100 and 300 These two toner components areadvantageously such that the toner or toners constituting the componentof one polarity at least are soluble in Water, alkalis or acids and/orwhen heated decompose to yield volatile substances or substances readilysoluble in these solvents.

Natural and synthetic resins such as colophony, copals, darnmar resin,asphalts, colophony-modified phenol resins, ketone resins, maleicresins, coumarone' resins, polyacrylic acid resins and polystyrenes aresuitable for the toner component which assumes a positive charge.Mixtures of these resins may also be used. Inorganic and/or organicpigments and/ or dyestuffs may be added to these resins to impart tothem a definite positive polarity. Examples of suitable substances arecarbon black, zinc oxide, titanium dioxide, barium sulfate, minium anddyestuffs such as those listed in Schultzs Farbstofitabellen, vol. 1,7th edition (1931). Also mixtures of such dyestuffs and/ or pigments maybe used.

Minute quantities of these substances, e.g. 0.5 percent by weight ofcarbon black, are often sufficient to impart a clearly positivecharacter to the resins. It is also often advantageous to incorporatesmall uantities of waxes and/or organic compounds of low melting pointand their substitution products, e.g. 0.1 to 10 percent, preferably 1 to5 percent by weight, to the positive toner component to influence in adesirable direction its melting point and adhesive power.

Waxes suitable for this purpose include natural waxes such as carnaubawax, beeswax, Japan wax, montan wax, ceresine and synthetic waxes suchas those marketed as A-wax, OP-wax, SPO-wax, V-wax, O-wax, E-wax, HardWaxH, Hard Wax W, various waxes known as Ruhrwachse, and particularlythe products available as Gersthofener waxes with the codings S, L, O,and OP.

Suitable organic compounds of low melting point include, in particular,substituted and unsubstituted aromatic compounds having melting pointsbetween 40 and C., for example naphthols, such as l-naphthol andZ-naphthol, aromatic compounds such as acenaphthene, acylamino compoundssuch as acetanilide, halogen aromatic compounds such asp-dibromobenzene, amino compounds such as 2,4-diamino toluene,o-phenylene diamine, phenols such as resorcinol, and diphenylamine andderivatives thereof.

Metal resinates are particularly suitable as the toner component whichacquires the negative charge. Metal resinates, also known as resinsoaps, are resin acid salts of metals of the first to eighth groups ofthe Periodic System.

Resinates, and mixtures thereof, of aluminum, barium, lead, calcium,cerium, iron, cobalt, copper, magnesium, manganese and zinc areparticularly suitable.

In addition to these metal resinates, the negative toner component mayinclude up to 50 percent by weight of r resins, such as those abovespecified for use in the positive toner component. The above mentionedwaxes and lowmelting organic compounds having melting points between 40and 150 C. may also be added to the metal resinates.

Dyestuffs and pigments, such as those described for use in the positivetoner component'may also be added. However, in general, the content ofpigments and dyestuffs should not exceed 10 percent of the metalresinates in order to assure preservation of the negative polarity,except in the case of dyestuffs containing metals, of which quantitiesof up to 50 percent, but preferably not more than 35 percent by weight,may be added.

Examples of dyestuffs containing metals are those which contain in themolecule metals such as copper, zinc, magnesium, iron, sodium orpotassium bound in complex form, e.g. complexes such as chlorophyll orcopper, zinc or magnesium phthalocyanine or Naphthol Green B. Alsouseful are double salts of dyestuff molecules, e.g. zinc chloride doublesalts. Complexes of heteropoly acids, such 3 asphosphoric/molybdic/tungstic acid with dyestuffs, may also be used.

Sulfonic acids and, in particular, carboxylic acids are suitable asalkli-soluble toners, e.g. oxalic acid, adipic acid, tartaric acid,'benzoic acid, aminobenzoic acid, chlorobenzoic acid, naphthalic acid,2-hydroxy-1-naphthoic acid, tetrachloropht-halic acid, anthraquinonecarboxylic acid, benzene sulfonic acid, chlorobenzene sulfonic acid,naphthalene sulfonic acid, naphthol sulfonic acid, naphthylaminosulfonicacids, aminonaphthol sulfonic acids, benzi- 'dine sulfonic acids,anthracene sulfonic acids, anthraquinone sulfonic acids; and also acidanhydrides such as naphthalic anhydride and pht halic anhydride and acidimides such as naphthalic imide and hydroxynaphthalic imide,sulfonamides such as toluene sulfonamide, naphthalene sulfonamide,naphthalene sulfoanilide and phenyl sulfanilide, imidazoles such asbenzimidazole, Z-mercaptobenzimidazole and2-(4-aminophenyl)-4,5-diphenyl-imidazole, hydroxy compounds such asresorcinol, 4-hydroxydiphenyl and 4-hydroxy-benzophenone, naphthols suchas l-naphthol, purpurogallin, pyrogallol, 2,3-dihydroxynaphthalene andS-hydroxy-acenaphthene, triazole compounds such as2,5-diphenyl-1,3,4-triazole and pyrazole compounds such as 3,5-diphenylpyrazole.

The alkali-soluble organic compounds may be used uncolored or they maybecolored wit-h the above-mentioned organic dyestuffs or organic orinorganic pigments.

Resins which are obtained by the polymerization of vinyl chloride withesters of fatty acids such as acetic acid,

propionic acid and butyric acid, and which also contain a certainproportion of unsaturated monocarboxylic acids such as crotonic acid andcinnamic acid or unsaturated dicarboxylic acids such as maleic acid,fumaric acid or itaconic acid can also be used as alkali-soluble toners.Such products are commercially available. Styrene and maleic acidinterpolymers and resins having a high acid number, e.g. over 150, andof which colophony forms the major part, can also be used.

Suitable acid-soluble toners are solid basic organic compounds such asphenylene diamine, benzidine, diphenylamine, naphthylamine, naphthylenediamine, 5,6-benzoquinoline, 5,6-benzoquinaldine, 4-chloroaniline,4,4'-diamino-benzophenone and those of the triazole, imidazole andpyrazole compounds above named as alkali-soluble compounds and which,because of the presence of an NH group, are also acid-soluble.

As the water-soluble toners, the salts of the alkali-soluble toners,particularly their alkali metal salts, are suitable, as also are thesalts of the acid-soluble toners, particularly with strong inoragnicacids.

Examples are: alkali metal salts of organic aliphatic, aromatic orheterocyclic carboxylic acids or sulphonic acids such as sodium acetate,lithium citrate, sodium/ potassium tartrate, sodium benzoate, the sodiumsalt of naphthalene-l-carboxylic acid, the disodium salt ofnaphthalene-1,5-disulfonic acid, the potassium salt ofanthracene-l-sulfonic acid, the potassium salt of fluorene dicarboxylicacid and the sodium salt of 1-phenyl-5methy1 pyrazole carboxylic acid,water-soluble sulfonimides such as benzoic acid sulfimide and dibenzenesulfonylimide. Lower, water-soluble organic carboxylic acids, diandtricarboxylic acid and hydroxycarboxylic acids such as oxalic acid,succinic acid, adipic acid, maleic acid, tartaric acid, malic acid,citric aid, salicyclic and resorcyclic acid and water-soluble resinssuch as polyvinyl alcohols, polyvinyl pyrrolidones and low molecularweight condensation products of melamine-formaldehyde resins can also beused.

Readily decomposable carboxylic acids, such as malonic acid, acetonedicarboxylic acid, citraconic acid and furfural malonic acid, inparticular, maybe used as toners which decompose under the influence ofheat.

The developer according to the invention may include at least oneorganic resin-type toner component which assumes a positive charge andat least one inorganic toner component which assumes a negative charge,the particle sizes being approximately equal.

Suitable inorganic toner components which assume a negative charge aresalts such as potassium sulfate, calcium sulfate, ammonium chloride,sodium chloride, potassium bromide, copper sulfate, aluminum/potassiumsulfate and sodium sulfate; oxides, e.g. iron oxide, titanium dioxide,zinc oxide, aluminum oxide and copper oxide; silicates, e.g. kieselguhr,silical gel, talcum and glass powder; borates, e.g. sodium mctab-orateand potassium borate; and carbonates such as calcium carbonate,magnesium carbonate and potassium carbonate.

The organic toner components are advantageously prepare as follows: thepulverized starting materials are ground together very finely; themixture is heated to melting and stirred until a high degree ofhomogeneity is attained and the melt is then cooled. Alternatively, thefusible starting materials maybe liquified'by heating. The othercomponents are then stirred in and the mixture is cooled. The resultantmass is finely ground and screened and, for the toner, screen fractionswith an average particle size of about 1 to 100 preferably of about 10to 30 are used.

The inorganic toner substances are likewise finely ground and screenedand average particle sizes of about 1 to 100 preferably 1 to 10 and/or10 to 20 are used. The toner component which acquires a negative chargemay be mixed with the one which acquires a positive charge inproportions of about 1:1 to 20:1 by weight. Approximately equalproportions of the two types of toner are preferable.

The developer may contain 1 part by weight of toner to 15-50 parts byweight of iron powder. Particularly good results are obtained withproportions of 1 part of toner to 15-25 parts by weight of iron powder.

A general principle is that with relatively fine iron powder, ofparticle size 50-100,, a larger quantity of toner is necessary than isrequired in a mixture with fairly coarse iron filings, e.g. of particlesize 300-400u.

To develop latent electrophotographic images, the developer may bewithdrawn from a trough by means of a rotating magnetic roller andpassed over the surface of the exposed photoconductive material. Theiron powder releases the toner and a visible image is formed, which maybe fixed.

The quality of the image is influenced not only by the proportions ofthe components of the developer and the particle size of the iron powderbut also by the type of magnetic applicator used. Advantageously,magnetic brushes which are as soft as possible are used, e.g. thoseconstituted by permanent or electrically excited bar magnets. These canbe arranged in a plane or in radial formation on a rotating roll.Magnetic rolls excited by electrical windings in grooves parallel to theaxis are also very suitable. Images free of background are obtained inwhich even very large solid image areas are homogeneously developed witha covering layer of developer. At the same time, the finest lines aresharply reproduced.

The invention Will be further illustrated by reference to theaccompanying drawings in which:

FIGURE 1 schematically illustrates a device for mag netic brushdevelopment using a bias electrode, and

FIGURE 2 schematically illustrates a device for magnetic brushdevelopment using a permanent electromagnetic roller.

Referring to FIGURE 1, a permanent magnet 1 has a plurality ofbrush-like bristles 2 of the developer mixture of the invention adheredthereto. A battery 3, or other source of biasing voltage, is connectedwith the permanent magnet 11 and a supporting plate 4 of highconductivity. An electrophotographic material 5, having aphotoconductive layer on the top thereof and a latent electrostaticimage on the photoconductive layer, is developed with positively chargedtoner in the image areas 6, and developed with the negatively chargedtoner in the image areas 6a.

Referring to FIGURE 2, a container 7 holds a quantity of .the developermixture 8 of the invention and a battery or other source of biasingvoltage 9 is connected with the axle of the permanent electromagneticroller 14 and the supporting plate 10 of high conductivity. Anelectrophotographic material 11, having a photoconductive layer on thedownwardly facing side and a latent electrostatic image on thephotoconductive layer, is developed in the image areas 12 withpositively charged toner and in the image areas 12a with negativelycharged toner. The toner is applied to the latent electrostatic image bymeans of the brush-like bristles 13 of the developer mixture of theinvention which are magnetically adhered to the roller 14.

If suitable photoconductors are used, particularly organicphotoconductors which can be charged both negatively and positively,positive images can be obtained from positive as well as from negativemasters with the same toner.

If the colored toner component of the developer acquires a positivecharge, negative charging of the layer and exposure thereof behind apositive master will cause toner to become deposited in the image parts.

To obtain a positive image from a negative master, on the other hand,the same photoconductive coating is positively charged and exposedbehind a negative master so that a latent image is first obtained inwhich the image parts are no longer charged while in the other parts thepositive charge is preserved to a greater or lesser extent. To developsuch a latent image, the colored, positively charged component of thetoner must be caused to settle in the charge-free parts. For thispurpose, a positive bias voltage, with respect to the conductive supportof the photoconductive layer, is applied to the magnetic brush. In theelectric field formed between the magnetic brush and the layer, thepositive toner particles are preferentially deposited on the charge-freeparts while the other toner component of the developer settles on thepositively charged parts of the latent image. The level of the biasapplied is determined by the potentials remaining after exposure in theimage parts and the imagefree parts. If too low a bias is selected,insufficient toner is deposited on the image parts.

In the development process just described, where the colored componentof the toner adheres to the image parts which are still charged afterexposure, images completely free of background are obtained. Exposuremust of course be adequate, because othe-rwiseparticularly if a masterof poor contrast has been used-residual charges will be left on theimage-free parts which may result in a disturbing background upon theapplication of toner. But even then, images completely free ofbackground can also be produced if a bias of the same polarity as thecharge on the coating is applied to the magnetic brush.

The processes for application of the toner that have just been describedcan of course also be employed-With a developer in which the coloredcomponent assumes a negative charge if charging and exposure conditionsare reversed. I

The above described advantageous results obtained when development iscarried out with the developer and process provided by the inventionacquire particular significance when the copy is to be further processedto a printing plate. For this purpose, the fixed copy is wiped over withacids or alkaline agents to remove the photoconductive coating and,where appropriate, the toner layer in the parts that are to be madewater-acceptant, and the support is bared. Consequently, substances .areused for the photoconductive coating which are soluble in acids oralkalis.

For example, in the case of an electrophotographic material, thephotoconductive coating of which is soluble in alkalis, and a developer,the'toner of which acquires a positive charge and is likewise soluble inalkalis, if a negative charge is applied to the photoconductive coating,a printing plate is obtained after development, fixing, and alkalinetreatment which is a reversed image of the master. If direct images arerequired, the coating is given a positive charge and the process is inother respects the same. If, with the same developer, an acid-solublesubstance is used as the toner which assumes a negative charge, thisdeveloper can also be used for electrophotographic material thephotoconductive coating of which is soluble in acids. If the coatingisnegatively charged, direct images are obtained, while withpositivecharging, reversed images are obtained.

After the electrophotographic image has been developed and fixed, it isconverted into a printing plate by treatment with an alkaline or an acidliquid, according to the solubility properties of the photoconductivecoating. In the parts which are to be made water-acceptant, according tothe type of development, the image parts or the image-free parts. of thephotoconductive coating, including any toner or decomposition productthereof that may be present thereon, are washed away and the support isbared. It is made water-acceptant either during this treatment or byafter-treatment. The support may be bared by wiping with the liquid,e.g. with a soaked cotton pad, or the plate may be immersed in a bath ofthe liquid, or rolls or other mechanical devices may be used to applythe liquid.

After the treatment with alkali or acid, the printing plate is rinseddown with water and inked up with greasy ink. When the plate has beenset up in a machine, long be produced.

If a toner mixture is used from which at least one toner can beeasily'removed during or after fixing, it is possible for this effect tobe exploited particularly in the case of printing plates. It is alsopossible for two toners with quite widely differing softening or meltingpoints to be used, e.g. with a difference in melting point'of 20 to 200C., preferably 50 to 150 C. If the toner of higher melting point is usedas the one which is later removed, this prevents broadening in thecontours of the toner of lower melting point, so that there is a furtherincrease in the sharpness of the printing plates. I

The developers described are advantageously used for the development ofprinting plates from which one toner, together with a part of thecoating, is to be removed. -It is also possible, particularly if onetoner is colorless, for any other electrostatic image to be developed.It is immaterial by what means the electrostatic image has beenproduced, e.g. by the action of visible light, X-rays, UV or infra-redlight on suitably sensitive layers or by direct electronic production ofthe image on an insulating layer or by the transfer of an electrostaticimage to an insulating layer.

The invention will be further illustrated by reference to the followingspecific examples:

Example 1 20 par-ts by weight of iron powder having a range of particlesize of IOU-150 are mixed with 1 part by weight of toner of averageparticle size 10-15 The toner use-d consists of two components:

(a) 10 parts-by weight of a powdered colorless interpolymer consistingof percent of vinyl chloride, 14 percent of vinyl acetate and 1 percentof maleic acid and (b) 3 parts by weight of a toner obtained from 3parts by weight of polystyrene of low melting point, 3 parts by weightof a m-aleinate resin, 0.3 part by weight of spirit-soluble nigrosineand 0.1 part by weight of Pigmen-t Deep Black, by melting theseconstituents together, grinding and screening.

The resultant developer, in which toner component (a) acquires anegative charge and toner component (b) acquires a positive charge, istaken up from a trough by means of an electrically excited magneticroll. The magnetic brush, constituted by the iron powder adhering to theroll and carrying the toner on its surface, is grounded.

This developer is brought, in this form, into contact with a latentelectrostatic image on the surface of a photoconductive insulating layersupported on an aluminum foil which had been charged before exposure to300 volts. The black, positively charged toner component is attracted bythe negatively charged image and the colorless, negative toner componentis repelled by the image but adheres to the uncharged edges of the blackimage. The image is fixed by heating to about 160 C. The resultantsmear-fast copy is free of background while the large-area solid imageparts are homogeneously colored.

To convert the image into a printing plate, it is wiped over with asolution of 10 percent monoethanolamine, percent sodium silicate and 85percent by weight polyethylene glycol. This alkaline solution dissolvesthe image-free parts of the coating and also the alkali-soluble toneradhering to the edges of the image. After a brief rinsing with water,the image is inked up with greasy ink and used for printing in an olfsetmachine. The plate has a long printing run and produces prints free fromdefects.

Example 2 25 parts by weight of iron powder having a range of particlesize of 50-100 produced by a spraying process, are mixed with one partby weight of toner of average particle size -l5,u.. The toner consistsof two components:

(a) 10 parts by weight of finely pulverized 5-chloro-2-methyl-benzimidazole and (b) 10 parts by weight of a mixture of 4 partsby weight of polystyrene, 3 parts by weight of colophony, 2 parts byWeight of carbon black and 1 part by weight of spirit-soluble nigrosine.

The developer so produced is used to develop a latent electrostaticimage on an aluminum foil carrying a photoconductive insulating layerwhich had been charged to -300 volts and exposed imagewise by thecontact process under a positive master and the image is fixed. A sharpand clearly defined image of the master is formed.

It the exposure were insufiicient, i.e. if the entire charge had notleaked away in the image-free parts, the latent electrostatic imagecould still be developed without background by means of this developerif a negative bias (about 80 volts) with regard to the support isapplied to the magnetic brush during development.

The fixed toner image can be converted into a printing plate by wipingover with a solution containing, by weight, 0.5 percentmonoethanolamine, 60 percent glycerine and 39.5 percent ethylene glycol.After being sprayed down with water, the image is inked up with greasyink, after which printing can be performed. With a printing plate ofthis type, long runs are possible and the prints reproduce fine linessharply and clearly.

Example 3 1 part by weight of a toner consisting of equal parts byweight of a finely pulverized 4-amino-anisole-Z-sulfonic acid and acolored interpolymer of vinyl chloride, vinyl acetate and maleic acidare mixed with 30 parts by weight of iron filings having an averageparticle size of 150- ZOO/1..

This developer is ,used as described in Example 1 and Example 2 todevelop a latent electrostatic image on a photoconductive insulatingcoating supported on an aluminum foil produced by contact exposurebehind a master after the insulator layer has been positively charged to130 volts.

ill

Q an After fixing, a full-tone background-free image is obtained whichcan be converted into a printing plate as described in Example 1 andExample 2.

Example 4 parts by weight of iron powder, produced by a spray processand subsequent grinding and having a particle size range of 50 to 100are mixed with 1 part by weight of a toner of an average particle sizeof 5l5 1.. The toner consists of two components:

(a) 10 parts by weight of kieselguhr and (b) 10 parts by weight of acolored resin mixture of polystyrene of low melting temperature,maleinate resin and Pigment Red B.

A photoconductive insulating coating on a paper support of adequateelectrical conductivity is charged to -340 volts and then imagewiseexposed by the contact process under a negative master. The resultantelectrostatic latent image is made visible by applying the developer bya magnetic brush which is given a negative bias of -300 volts. Theresultant image is free of background. The finest line drawings areexactly and sharply reproduced. Also, large solid areas of 100 cm. andmore can be covered completely homogeneously and uniformly With tonerparticles.

Example 5 1 part by weight of the toner mentioned in Example 2, having aparticle size of 10 to is mixed with parts by weight of iron filingshaving a particle size of 100 to 200 This developer is applied by amagnetic applicator to a latent electrostatic image, produced by thereflex process, on a zinc oxide coating supported on a paper foil. Thedeveloped toner image can be transferred to another surface to produce acopy which is a sharp and background-free reproduction of the master.

It will be obvious to those skilled in the art that many modificationsmay be made within the scope of the present invention without departingfrom the spirit thereof, and the invention includes all suchmodifications.

What is claimed is:

1. A method of developing an electrostatic latent image which comprisesapplying to the image a developer comprising a mixed toner thecomponents of which are powders having approximately equal particlesizes, one component being capable of acquiring a positive charge andconsisting essentially of resin, and the other component being capableof acquiring a negative charge and consisting essentially of aninorganic substance, and an iron powder carrier, and fixing thedeposited resin powder.

2. A method of developing an electrostatic latent image which comprisesapplying to the image developer comprising a mixed toner the componentsof which are powders having approximately equal particle sizes, onecomponent being capable of acquiring a positive charge and consistingessentially of resin, and the other component being capable of acquiringa negative charge and consisting essentially of an inorganic substance,and an iron powder carrier, by means of a magnetic device which servesalso as a bias electrode, and fixing the deposited resin powder.

3. A method according to claim 2 in which the toner component capable ofacquiring a positive charge is a mixture of a resin and a wax.

4. A method according to claim 2 in which the toner component capable ofacquiring a positive charge is a mixture of a resin and an organiccompound having a melting point in the range of about l50 C.

5. A method according to claim 2 in which the toner component capable ofacquiring a negative charge is selected from the group consisting ofinorganic silicates, metal oxides, and inorganic metal salts andinorganic ammonium salts.

6. A method according to claim 2 in which the toner component capable ofacquiring a charge of one polarity is soluble in a liquid.

7. A method according to claim 6 in which the soluble I component isselected from the group consisting of sulsoluble component is an alkalimetal salt of an acid selected from the group consisting of sulfonic andcarboxylic acids.

12. A method according to claim 2 in which one toner component capableof acquiring a charge of one polarity decomposes upon heating to amaterial soluble in a liquid.

13. A method according to claim 2 in which the toner component capableof acquiring a negative charge is mixed with the toner component capableof acquiring a positive charge in proportions of about 1:1 to :1 byweight.

14. A method according to claim 2 in which the developer contains onepart by weight of toner to 15-50 parts by weight of iron powder.

15. A method according to claim 2 in which the resin is in admixturewith a dyestufl.

16. A method according to claim 2 in which the resin is in admixturewith a pigment.

17. A method for the preparation of a printing plate which comprisesdeveloping an electrostatic latent image on a supported photoconductivecoating by contacting the image with a developer comprising a mixedtoner the components of which are powders having approximately equalparticle sizes, one component being capable of acquiring a positivecharge and consisting essentially of resin, and the other componentbeing capable of acquiring a negative charge and consisting essentiallyof an inorganic substance, and an iron powder carrier, fixing thedeposited resin powder and removing the photoconductive coating from thesupport in the resinfree areas.

18. A method for the preparation of a printing plate which comprisesdeveloping an electrostatic latent image on a supported photoconductivecoating by contacting the image with a developer comprising a mixedtoner the components of which are powders having approximately equalparticle sizes, one component being capable of acquiring a positivecharge and consisting essentially of resin, and the other componentbeing capable of acquiring a negative charge and consisting essentiallyof an inorganic substance, and an iron powder carrier, by means of amagnetic device which serves also as a bias electrode, fixing thedeposited resin powder and removing the photoconductive coating from thesupport in the resin-free areas.

19. An electrophotographic developer comprising a mixed toner thecomponents of which are powders having approximately equal particlesizes, one component being capable of acquiring a positive charge andconsisting essentially of resin, and the other component being capableof acquiring a negative charge and consisting essentially of aninorganic substance, and an iron powder carrier.

20. An electrophotographic developer according to claim 19 in which thetoner component capable of acquiring a positive charge is a mixture of aresin and a Wax.

21. An electrophotographic developer according to claim 19 in which thetoner component capable of acquiring a positive charge is a mixture of aresin and an organic compound having a melting point in the range ofabout 40-150 C.

22. An ele-ctrophotographic developer according to claim 19 in which thetoner component capable of acquiring a negative charge is selected fromthe group consisting of inorganic silicates, metal oxides, and inorganicmetal salts and inorganic ammonium salts.

23. An electrophotographic developer according to claim 19 in which onetoner component capable of acquiring a charge of one polarity is solublein a liquid.

24. An electrophotographic developer according to claim 23 in which thesoluble component is selected from the group consisting of sulfonic andcarboxylic acids.

25. An electrophotographic developer according to claim 23 in which thesoluble component is a conjoint polymer of a vinyl halide with an esterof a fatty acid and an unsaturated carboxylic acid.

26. An electrophotographic developer according to claim 23 in which thesoluble component is a solid, basic organic compound. 4

27. An electrophotographic developer according to claim 23 in which thesoluble component is selected from the group consisting of imidazoles,hydroxy, aromatic compounds, triazoles and pyrazoles.

28. An electrophotographic developer according to claim 23 in which thesoluble component is an alkali metal salt of an acid selected from thegroup consisting of sulfonic and carboxylic acids.

29. An electrophotographic developer according to claim 19 in which onetoner component capable of acquiring a charge of one polarity decomposesupon heating to a material soluble in a liquid.

30. An electrophotographic developer according to claim 19 in which thetoner component capable of acquiring a negative charge is mixed with thetoner com-,

ponent capable of acquiring a positive charge in proportions of about1:1 to 20:1 by weight.

31. An electrophotographic developer according to claim 19 containing 1part by weight of toner to 15-50 parts by Weight of iron powder.

32. An electrophotographic developer according to claim 19 in which theresin is in admixture with a dyestulf.

33. An electrophotographic developer according to claim 19 in which theresin is in admixture with a pigment.

34. A method according to claim 2 in which the magnetic device is arotating magnetic roller.

References Cited by the Examiner UNITED STATES PATENTS 2,297,691 10/1942 Carlson 117l7.5 X 2,618,551 11/1952 Walkup 1l7-17.S X 2,786,4403/1957 Giaimo l18637 2,851,373 9/1958 Tregay et al. l17--17.5 2,874,0632/1959 Greig 118-637 X 2,880,696 4/1959 Clark et al. 11717.5 X 2,890,9686/1959 Giaimo 11717.5 X 2,919,247 12/1959 Allen 252-621 2,986,521 5/1961Wielicki 11717.5 X 3,013,890 12/1961 Bixby l1717.5 3,041,165 6/1962 Suset al 96-1 3,060,051 10/ 1962 Johnson et al.

3,083,117 3/1963 Schmiedel et al. 117l7.5 3,093,039 6/ 1963 Rheinfrank11717.5 X 3,165,420 1/ 1965 Tomanek et al. 252-621 X FOREIGN PATENTS203,907 11/1956 Australia. 608,901 11/1960 Canada.

WILLIAM D. MARTIN, Primary Examiner. G. L. HUBBARD, Assistant Examiner.

1. A METHOD OF DEVELOPING AN ELECTROSTATIC LATENT IMAGE WHICH COMPRISESAPPLYING TO THE IMAGE A DEVELOPER COMPRISING A MIXED TONER THECOMPONENTS OF WHICH ARE POWDERS HAVING APPROXIMATELY EQUAL PARTICLESIZES, ONE COMPONENT BEING CAPABLE OF ACQUIRING A POSITIVE CHARGE ANDCONSISTING ESSENTIALLY OF RESIN, AND THE OTHER CM-